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Have you ever launched a long-running command, only to realise, oops, there's actually more I wanted to do? Now you have to open a new instance of your terminal, cd into the directory you were in, it's a whole hassle. But it doesn't have to be! Using jobs, you can manage multiple concurrent processes from a single shell.

Terminology

• Job: A process that is spawned by a shell
• Running: A job that is actively doing work
• Stopped: A job that has been paused, and may resume
• Terminated: A job that has been terminated, and may not resume
• Disowned: A job that was spawned by a shell, but is no longer visible to it

Spawning a job in the foreground

Jobs are spawned in the foreground by default when a command is run. The terminal will attach to the job and detach when the job is completed or terminated.

Spawning a job in the background

Jobs can be put into the background immedately by using & at the end. For example, updatedb & will run updatedb in the background.

You can add more commands directly after &, and they will run as normal. For example, updatedb & echo "updatedb started".

To run multiple commands in sequence in the background, there are two options: wrap them in () to create a sub-shell (so variable changes will not persist in this scope), or in {} to create a command group (so changes do persist). For example:

( updatedb; echo "updatedb complete"; ) & echo "updatedb started"

Listing jobs

The jobs command presents a list of jobs beloning to this terminal.

[1]   Running                    sleep 10
[2]-  Stopped                    sleep 100
[3]+  Stopped                    sleep 1000

The number in the brackets is the job ID. Once a job is assigned an ID, its ID will never change. The ID will always be the next number after the highest ID of a running or stopped job (starting at 1 for the first job).

+ denotes the current job, that is, the default for fg and bg to act upon. - is the previous job, next in line to become the current job.

The text to the right is the invocation of the command as it appeared in the shell.

Stopping a foreground job

For jobs in the foreground, use CtrlZ to immediately stop the job. This will output the job to the console:

[1]+   Stopped                    sleep 10

Stopping a background job

To stop a background job, use kill -STOP %i, where i is the job ID (required).

Resuming a stopped job

fg %i will bring a job to the foreground and resume it if it is stopped. If %i is omitted, it implies the most recently stopped job.

To resume a stopped job in the background, use bg %i - again, omitting %i implies the most recent job. This will output the job line.

Monitoring background jobs

A job in the background will report its status after completion after the next time the terminal flushes its input; use set -b to report immediately (set +b to restore behaviour).

Terminating a job

To terminate a foreground job, use CtrlC. This will simply stop the job.

To terminate a background job, use kill %i. You can send a signal from 1-9 using kill -n %i if the initial kill doesn't work. This will the job line.

Disowning a job

To disown a job, use disown %i.

By default, closing the terminal will still send SIGHUP (the hang-up signal), which some programs may take as an indicator to exit; add the -h flag to avoid this.

Notes on output streams

The job lines output by bg and kill aren't sent to /dev/stdout or /dev/stderr. They go directly to the terminal and cannot be redirected.

Breaking it all

set +m disables job control. & syntax and kill still work, but CtrlZ, fg and bg all stop working.